1. Field of the Invention
The present invention relates to a semiconductor optical device and method for manufacturing the same, which can be used for optical information processing and optical communication and the like, suitably for a pumping light source for a fiber amplifier.
2. Description of the Related Art
A light source for optical information processing and optical communication is required to have high power output and high reliability, particularly in case of a pumping light source for a fiber amplifier which can be used for an optical repeater in a submarine optical cable, a semiconductor optical device is required to have a long lifetime and high reliability.
As the pumping light source for a fiber amplifier, the semiconductor optical device having an emission wavelength of 0.98 μm or 1.02 μm is generally selected and a strained quantum well structure is adopted for an active layer thereof. For example, there is a distortion of about 1% between an InGaAs quantum well layer and a GaAs guide layer.
For main degradation causes of the semiconductor optical device, end face degradation due to optical absorption on an optical exit face and internal degradation due to dislocation in a crystal or distortion between epitaxial growth layers are known.
To counter the end face degradation, a window structure in which the band gap energy of the optical exit face is larger than that of the active layer can be adopted to prevent the optical absorption or an appropriate coating on the end face can be designed.
To counter the internal degradation, a substrate having a low dislocation density can be used or an active layer with a strain-compensated structure can be adopted.
The related prior arts are listed as follows:
[Document 1]
G. Beister et al., “Monomode emission at 350 mW and high reliability with InGaAs/AlGaAs (λ=1020 nm) ridge waveguide laser diodes”, ELECTRONICS LETTERS, 16 Apr. 1998, Vol. 34, No. 8, pp. 778-779
[Document 2]
Toshiaki Fukunaga et al., “Reliable operation of strain-compensated 1.06 μm InGaAs/InGaAsP/GaAs single quantum well lasers”, Appl. Phys. Lett., Vol. 69(2), 8 Jul. 1996, pp. 248-250
In FIG. 2 of the above-mentioned document 1, the result of a reliability examination of the semiconductor optical device of 1.02 μm wavelength band is illustrated using graphs. The examination conditions are atmosphere at 40 degree-C., a constant output of 300 mW and a measurement size of ten. The horizontal axis shows an aging time by 1,000 hours, and the vertical axis shows a driving current (mA).
These graphs show initial degradation for three of ten samples in which the driving current rapidly increases in an initial stage. For the remaining seven samples, the driving current gradually increases with time progress and a degradation rate is calculated by linear approximation with 1.5×10−5 to 8.6×10−5 (/h), resulting in 1.5% to 8.6% of increase in current at 1,000 hours.
In FIG. 6 of the above-mentioned document 2, the result of a reliability examination of the semiconductor optical device of 1.06 μm wavelength band is illustrated using graphs. The examination conditions are atmosphere at 25 degree-C., a constant output of 250 mW and a measurement size of both ten of SC-SQW (strain-compensated single quantum well) lasers and ten of SL-SQW (superlattice single quantum well) lasers. The horizontal axis shows an aging time by 1,000 hours, and the vertical axis shows a driving current (mA).
These graphs show that all of the SL-SQW lasers are degraded before 1,000 hours, on the other hand, all of the SC-SQW lasers are not remarkably degraded because the driving current does not increase even at 1,000 hours.